1. Technical Field
The disclosure relates to an indicator and method for fabricating the same, and in particular relates to a Ni-containing indicator and method for fabricating the same.
2. Description of the Related Art
A humidity indicating card (or a humidity indicator) is used for many purposes including, determining the relative humidity in shipping containers and packages, such as packaged electronics and telecommunication products. The humidity indicator cards are typically impregnated with a humidity sensitive composition and dried. Most humidity indicating cards made today use cobalt chloride as the indicator, which changes color based on the level of hydration of the cobalt chloride. Cobalt chloride is, however, a very toxic chemical compound and the European Union classified cobalt chloride as a category 2 carcinogen (Council Directive 67/548/EEC). In the Commission Directive 2004/73/EC (Apr. 29, 2004), cobalt chloride is listed as a chemical substance which may cause cancer by inhalation at concentrations of 100 ppm or higher. Thus, cobalt chloride humidity indicators have limited applications, and it is necessary to find alternate materials to replace cobalt chloride as a humidity-indicating medium.
U.S. Pat. No. 2,460,072 and U.S. Pat. No. 2,460,067 disclose using copper(I) chloride and copper(II) bromide, respectively, but these compounds are not considered suitable candidates for a commercial silica gel-based humidity indicator because of potential toxicity and environmental considerations.
U.S. Pat. No. 6,663,679 discloses a device for monitoring humidity, including a first layer, which is a visual indicator, and a second layer covering and obscuring the first layer, wherein the second layer includes a deliquescent material. The deliquescent material picks up moisture and dissolves itself in the moisture, becoming transparent and exposing the first layer. However, the device is a non-reversing humidity indicator.
U.S. Pat. No. 6,753,184, and U.S. Pat. No. 6,927,063 disclose iron salts as an alternate humidity-indicating media. These humidity-indicating media, however, have a color change that is not very distinctly detectable.
There remains a need for an improved humidity indicator that provides easy detectable color changes and is non-toxic.